Patent GB-1210721-A (GB)
Title: Improvements in or relating to railway vehicle propulsion control systems
Inventor: JACKSON DEREK CHARLES
IPC: B60L50/12
Filed: 1968-10-17
Abstract: 1,210,721. Automatic speed control. BRITISH RAILWAYS BOARD. Dec. 12, 1969 [Oct. 17, 1968], No.49247/68. Heading G3R. A diesel-electric railway vehicle has a propulsion system in which a plurality of D.C. electric motors 4 are supplied with power from a generator 2 driven by an engine 1, the fuel supply to the engine and the motor and generator currents being controlled to maintain constant speed set by a controller 18. Three error signals e 1 , e 2 , e 3  are used in the control system. These respectively represent the vehicle speed error; the difference between the generated power P+Pa (Pa being the power developed by auxiliary generators 22 supplying heating, &c. ) and the desired power Pd; and the difference between the maximum permissible motor armature current Im and the largest actual current f. Each motor 4 has its armature connected in parallel across the generator and its field energized by a controlled rectifier unit 8m (not described) which has a first feedback loop Fig. 3 (not shown) arranged so that the field current is maintained equal to a desired value Ifd set by a main controller 12 and also a second delayed feedback loop which seeks to maintain all the motor armature currents equal by adjusting the field currents. The generator field is controlled by a controlled rectifier unit 9g to maintain the current equal to a desired value Ifgd set by the controller 12. Associated with the motors is a rheostatic braking system comprising a contactor 6 energized in response to a control signal from the controller 12 to disconnect the motors from the generator and connect them in parallel across an energy dissipating resistor Figs. 1 12 (not shown). The controller 12 receives signals representing the desired train speed (Ud) and actual train speed (U); the auxiliary power (Pa); engine speed (n) and generator voltage (Vg). It also receives signals representing the motor armature currents (I 1 -I 6 ) from which it selects the largest (I) and also generates a signal (#I) representing the difference between the largest and smallest. From the speed signals (U, Ud) it generates the speed error signal e 1  which is used to adjust the governor setting of the engine to alter its speed. From the speed of the engine is determined the value Pd for the desired power. From the voltage (Vg) and the largest current (I) is generated a value for P, the power consumed; from these is generated the second error signal e 2 . A function generator 25 provides the maximum current signal Imwhich is made equal to the rated starting current for the motors while the generator is below half rated voltage and falls towards rated continuous current as generator voltage rises Fig. 5 (not shown). Im is reduced to zero when the set speed signal Ud is reduced to zero or when the difference in armature currents (#I) is excessive, e.g. during wheel slip; changes in Im are controlled by a function generator 25 so that it increases or decreases exponentially. During driving, the signal Im serves as the desired current signal Id and is compared with the largest armature current (š) to provide the third error signal e 3 . A switching device 26 determines which of e 2  or e 3  is applied to an integrator 28 while a device 27 causes the output of the integrator 28 to control the generator field until it reaches full excitation and thereafter to control the motor fields. Four modes of operation of the control system are identified, three driving and one braking. At starting the set speed signal Ud is increased from zero; the speed error signal e, is produced and increases the engine speed towards maximum. At this stage the power consumed by the motors (P) is low and therefore e 2  is positive. Switch 26 therefore selects e 3  (mode 1) so that the generator field is controlled to prevent the armature currents rising above Im which increases exponentially towards rated starting current; if wheel slip occurs Im is reduced until it ceases; this is repeated Fig. 10 (not shown) until the full starting current is reached. When generator voltage reaches half maximum Im is reduced to the continuous rating; e 3  therefore becomes positive. As the motors 4 accelerate the power consumed (P) eventually exceeds the power available from the engine (Pd) and therefore e 2  becomes negative. Switch 26 therefore selects e 2  and the generator field is controlled to keep P+Pa=Pd (mode 2). As train speed increases generator field energization must be increased to maintain the power consumption. Eventually the generator field is fully energized and thereafter the motor fields are weakened (mode 3). If speed of the train is excessive, i.e. speed error signal e 1  exceeds a pre-set negative value, a switching device 24 energizes the brake contactor 6 to apply rheostatic braking; it also changes over a switch 23 so that the speed error e 1  becomes the desired current signal (Id), and the switch 26 so that motor currents and therefore the braking are controlled to maintain constant speed. The braking is discontinued when e1 exceeds a pre-set positive value.